In certain applications it is sometimes necessary to dispense liquids out of a cartridge or similar container and onto a desired target. For example, hot melt adhesives such as PUR adhesive material may be dispensed out of a syringe-like cartridge and onto a desired target. One type of conventional cartridge or syringe dispensing system for dispensing hot melt adhesives typically operates as a contact dispenser by contacting the substrate directly with the adhesive exiting the nozzle. Another type of conventional hot melt dispensing system is operable to dispense beads or large droplets of hot melt adhesive in a non-contact manner.
In some applications such as cell phone assembly, the adhesive must be accurately dispensed into small grooves having widths of 0.5 millimeters and smaller. Furthermore, these grooves are located adjacent to microelectronics components or other elements which must be isolated from the adhesive. The conventional contact syringe dispensers for hot melt adhesives are generally not effective in these applications because the nozzle outlet cannot be moved close enough in a contact dispensing process for the dispensed adhesive exiting the nozzle to contact the small grooves without also inadvertently contacting surrounding elements. To accommodate such a small target area, it is desirable to dispense small-diameter droplets of adhesive in a controlled non-contact dispensing process. However, conventional non-contact hot melt dispensing systems do not produce a small enough droplet of hot melt adhesive to fit into the small grooves.
Conventional jetting dispensers have been used for dispensing reactive two-component materials, such as epoxies. See U.S. Pat. No. 5,747,102 to Smith et al., and U.S. Pat. No. 6,253,957 to Messerly et al. “Jetting” in the context of this specification is understood to mean rapidly dispensing minute amounts of viscous material such that each jetted droplet releases from the dispenser. Conventional jetting dispensers work well for their intended purpose. However, conventional jetting dispensers have not been used effectively to dispense small or minute droplets (i.e., less than 0.5 millimeters in diameter) of highly cohesive hot melt adhesives, including PUR adhesives because the droplets passed through the valve orifice do not acquire an adequate velocity during dispensing to effectively jet. In this regard, the highly cohesive hot melt adhesive sometimes fails to release from the nozzle. As a result, the nozzle becomes blocked with adhesive that tends to rapidly cure or solidify, which renders the entire dispenser inoperable. Moreover, attempts to jet hot melt adhesive with conventional jetting dispensers has resulted in premature wear or failure of the valve needle and actuation piston as a result of the high forces required to dispense and release hot melt adhesive.
The assembly of cell phones and other electronic devices can be a relatively difficult and slow process when compared to other hot melt adhesive assembly operations. As a result, the “open time” or amount of time when the adhesive is within a temperature range conducive to forming bonds necessarily must be increased for certain electronic device assemblies. While raising the temperature of the hot melt adhesive is one option for increasing the open time, hot melt adhesives are generally highly sensitive to high temperatures and degradation of the hot melt adhesives at these higher temperatures is possible. Thus, there is a limit on how much open time can be provided for favorable bonding of components with hot melt adhesive.
There is a need, therefore, for methods and jetting dispensers that address these and other problems.